In general, used as an apparatus that prints a color image on a sheet as a medium is a color tandem electrophotographic device having built-in multiple image forming units storing developers (for example, toners) of different colors from one another (for example, see Patent Document 1). FIG. 1 is a cross-sectional view that schematically shows the structure of an image forming apparatus 1 as a conventional color image forming apparatus. The image forming apparatus 1 shown in FIG. 1 is an intermediate transfer type image forming apparatus that is provided with a transfer belt 2 as a developer image carrier and a secondary transfer part 3, and comprises multiple image forming units 4a, 4b, 4c, and 4d storing developers of different colors from one another. The image forming apparatus 1 has a sheet P forwarded from a sheet feeding part 5, transfers developer images formed on photosensitive drums 6a, 6b, 6c, and 6d of the image forming units 4a, 4b, 4c, and 4d to the transfer belt 2 in the primary transfer parts 7a, 7b, 7c, and 7d, and transfers the developer images carried on the transfer belt 2 to the sheet P in the secondary transfer part 3. The sheet P, to which the developer images are transferred, is carried to a fuser part 8, and the developer images are fused to the sheet P by the developer images being heated and pressurized in the fuser part 8. The sheet P, to which the developer images are fused, is ejected to the outside of the image forming apparatus 1 through an ejection part 9. The image forming apparatus 1 can selectively switch between color printing and monochrome printing by a controller (not shown). When performing monochrome printing, the image forming units used for color printing can retreat to non image forming positions separated from the transfer belt. By having the image forming units that are not used for monochrome printing retreat to the non-image forming positions, wear of the image forming units due to contacting with the transfer belt can be prevented. In FIG. 1, as an example, these image forming units 4a-4c for color print are circled with a dot-dash line and the direction towards the non image forming positions are indicated with three upward arrows.